Highly selective asymmetric membranes for gas separation

ABSTRACT

A highly selective asymmetric membrane for gas separation is provided which comprises rigid aromatic polyimide or copolyimide polymers comprising the monomers diphenyldi(trifluoromethyl)methane 3,4,3&#39;,4&#39;-tetracarboxylic acid adianhydride and a monoaromatic metadiamine or a mixture of monoaromatic metadiamines.

The present invention concerns highly selective membranes for gasseparation using gas permeation. More specifically, the inventionconcerns asymmetric fluorinated polyimide membranes for gas separationand a method for the manufacture of said membranes. The membranes of theinvention are particularly effective for decarbonising natural gas.

Certain fluorinated aromatic polyimides, more precisely polyimidescontaining diphenyldi(trifluoromethyl)methane 3,4,3',4'-tetracarboxylicacid dianhydride (hereinafter termed "6F dianhydride") as thedianhydride portion and a diamine containing one or more aromatic ringsas the diamine portion, are known to have high permeabilities to certaingases and much lower permeabilities to other gases in their vitreousstate. These polymers are, then, both highly permeable (to certaingases) and highly selective. Permeability measurements in pure gaseshave shown, for example, that some fluorinated aromatic polyimides arefar more permeable to hydrogen, carbon dioxide or water vapour than tomethane and considerably more permeable to oxygen than to nitrogen.Separation potentials of a polymer can be approximated from thepermeability ratios measured in pure gases, termed the theoreticalseparation factors, and the initial results can then be improved by(more laborious) measurements in mixtures of gases. The observedselectivity in gas mixtures can be lower than the theoretical separationfactor, particularly when one constituent of the mixture plasticises thepolymer and thus facilitates transport of another gas. Carbon dioxideand water vapour, for example, tend to plasticise some aromaticpolyimides. The gas permeability of a polymer is sensitive totemperature: permeability increases and selectivity decreases withincreasing temperature. For some applications, gas permeation is carriedout at temperatures well above room temperature, for example 50° C. to60° C., in order to purify crude natural gas. Some aromatic polyimidesderived from 6F anhydride have a higher than average selectivity andretain a large proportion of that selectivity at relatively hightemperatures such as those just described. These highly selectivepolyimides are characterised by a very high glass transition temperatureof the order of 300° C. or more, combined with very reduced chainflexibility. Manufacturing gas permeable membranes from such polyimidesis highly desirable but they are difficult to manufacture by this methoddue to their low solubility in organic solvents, particularly thevolatile solvents used to make high flux asymmetric membranes. Thestructure of these membranes consists of a thin, dense skin supported bya much thicker, porous substructure. The dense skin contributes theselectivity and the substructure contributes the mechanical resistance.Since the permeability of a membrane is in practice determined by thatof the dense skin, it must be as thin as possible, for example of theorder of a fraction of a micrometer; the substructure may, however, beas much as a fraction of a millimetre thick. The membranes can be in theform of flat sheets of hollow fibres. The asymmetric structure isnormally produced by bringing one face of a film or capillary extrudateof polymer solution into contact with a liquid coagulant, ie, a liquidwhich is miscible with the solvent of the polymer but not in itself asolvent for the polymer. Before exposure to the coagulant, the facefirst superconcentrated in polymer, preferably by limited evaporation ofthe solvent. On contact with the coagulant, the solution gels on thesurface and the remainder of the volume then divides into two dispersedphases, one polymer-rich, the other polymer-depleted and coagulant-rich.Elimination of the solvent and coagulant produces the asymmetricmembrane. The permeability, selectivity and mechanical resistance of themembrane depends on the quality of the initial polymer solution. Thesolution filterability is an indication of its degree of homogeneity andstability. A solution which cannot be filtered through a filter with aretention limit of 0.5 micrometers, or at worst 1 micrometer, isconsidered to contain a high degree of coarse aggregates or to be in theprocess of becoming a gel.

Given the low solubility of the most selective (ie., the most rigid)polyimide derivatives of 6F dianhydride, the use of species which areslightly more flexible due to a diamine portion that allows local chainmovements have been proposed. However, solubility is improved at theexpense of the polymer selectivity. Thus relatively flexible polyimidesobtained using methylenedianiline and oxydianiline generally haveselectivities of about 25 and 30 respectively in CO₂ /CH₄ mixtures at55° C., while a more rigid polyimide obtained using metaphenylenediaminecan, for example, have a selectivity of 44. This latter polyimide hasbeen shown to have a selectivity of more than 50 under certainconditions. The gains in selectivity which can be expected when makingmembranes from more rigid polyimides can thus be seen.

One of the objects of the present invention is to provide asymmetricmembranes constituted by polyimides which are sufficiently rigid toensure high selectivities in gases, even at temperatures well above roomtemperature. A further object of the present invention is to provide amethod of manufacture of said rigid polyimide asymmetric membranes forgas separation.

The membranes of the invention are constituted by polyimides orcopolyimides wherein the dianhydride portion is 6F dianhydride withformula I: ##STR1## and the diamine portion is 1,3-diaminobenzene ormetaphenylenediamine, abbreviated to mPDA, with formula II: ##STR2##

Alternatively, 1,3-diaminobenzene can be partially or completelyreplaced by one or more derivatives which are mono- or dimethylated onthe aromatic ring. The membranes may be optionally coated with a layerof silicone elastomer on the skin side to protect it and seal anymicroperforations, in common with known practice.

The membranes of the invention, constituted as indicated above, havetheoretical separation factors (permeance ratios measured in pure gas)of about 70 to 110 for the CO₂ /CH₄ couple at room temperature, andeffective selectivities (permeance ratios measured in a mixture ofgases) of about 50 to 90 for the same CO₂ /CH₄ couple at roomtemperature.

Fluorinated polyimides for manufacturing the membranes have glasstransition temperatures of at least 250° C. and inherent viscosities ofat least 0.3 (measured in solution in N-methylpyrrolidone, containing1.0 gram of polymer per 100 ml of solvent). They can be prepared usingconventional polyimide synthesis methods. However, polyimides havingboth relatively high molecular weights and acceptable solubilities aremore easily produced by a chemical method in which polyamic acid iscyclised by addition of a dehydrating agent (for example, aceticanhydride plus triethylamine).

The invention concerns flat membranes and hollow fibres. In a preferredembodiment, a flat membrane is mechanically reinforced by a commerciallyavailable woven glass fabric or by a commercially available nonwovenpolyester fabric.

The method of manufacture of the membranes of the invention isprincipally characterised by the association of a particular type ofsolvent and a particular type of coagulant which is specially adaptedfor the polyimides used. The operating conditions of the methodobviously differ depending on whether flat membranes or hollow fibresare to be produced. The method consists in:

a) dissolving one or more specific polyimides of the invention, to atotal polymer concentration of between 10% and 35% by weight withrespect to the solution, in a solvent comprising at least 85% by weightof 1,4-dioxane and 0 to 15% by weight of N-methyl 2-pyrrolidone,N,N-dimethylacetamide and/or gamma-butyrolactone;

b) applying a film of the solution obtained to a support or extruding ahollow liquid filled fibre, using an annular die;

c) evaporating a portion of the solvent;

d) bringing the film or hollow fibre into contact with a liquidcoagulant constituted by a mixture of acetic acid and water comprising1% to 85% by weight of water, small quantities of other organic acidsbeing optionally added to the mixture;

e) rapidly drying the film or hollow fibre, which has been preferablywashed with water, methanol or ethanol; optionally

f) heat treating the film or hollow fibre; and optionally

g) coating the membrane skin with a thin layer of silicone elastomer toprotect against abrasion and, if necessary, to seal microfissures ormicroperforations.

The method is normally carried out at room temperature, with the obviousexception of heat treatment step f).

The various steps of the method will be described in more detail below.The operations are carried out at room temperature, except whereotherwise stated.

The polyimide(s) is/are dissolved in a solvent mixture which has beenprepared in advance. The major solvent, 1,4-dioxane, and thecomplementary solvents, N-methyl 2-pyrrolidone, N,N-dimethylacetamideand/or gamma-butyrolactone, are preferably dehydrated before use using3A molecular sieve. The solution may have a polymer concentration of 12%to 35% by weight, preferably 12% to 30% for a flat membrane and 15% to35% for a hollow fibre. A concentration sufficient to produce a solutionwith a viscosity of around a hundred poises reduces the risk of rupturein the newly formed fibre. However, solutions with a concentration ofmore than 35% by weight have a tendency to produce membranes with lowpermeability. The polymer solution is filtered to 0.5 and, if possible,to about 0.2 micrometer.

The flat membrane is produced by using a bar or blade to apply thesolution to a support which is preferably constituted by a woven ornon-woven fabric whose uncoated face is protected against subsequentcontact with the coagulant by a detachable polyolefin film. The woven ornon-woven fabric will remain attached to the membrane to reinforce itmechanically. If the temperature selected for heat treatment is notabove about 200° C., a non-woven polyester fabric can be used. Abovethis, a commercially available woven glass fibre fabric sized with asilane adherence enhancer, for example phenyltriethoxysilane, is used.The polymer solution deposited is preferably between 50 and 300micrometers thick.

The hollow fibre is produced by extruding the polymer solution throughthe annular orifice of a die which also comprises a central circularorifice through which a fibre core-filling liquid or central liquid isinjected. The two orifices are separated by a tubular wall with anegligible thickness at its extremity. The diameter of the centralorifice and the width of the annular space are preferably substantiallyequal and between 0.05 and 0.5 millimetre. The central liquid ispreferably a mixture of solvent and antisolvent in proportions whichproduce slightly coagulating properties in the mixture. The preferredflow rate of the liquid is about one eighth the flow rate of thesolution.

At a distance of about 5 to 10 millimetres from the knife or dieorifice, the film or extrudate is advantageously exposed to a current ofnitrogen which results in superficial superconcentration of the polymerby solvent evaporation. The gas stream is guided towards the surface ofthe bath and released about 5 millimetres therefrom. Even if the flowrate and the nitrogen stream cannot be fixed independently of theexposure time of the film or hollow fibre to the stream and vice-versa,there is some latitude in the choice of operational conditions. In apreferred embodiment of the method, the evaporation time is between 0.3and 1 second and the gas stream is 0.5 to 1.5 centimetres thick at anaverage speed of 2 to 10 centimetres per second.

The film or hollow fibre is subsequently immersed in a coagulant bathwhere it remains for at least 4 seconds, preferably between 5 and 20seconds. The coagulant is constituted by a mixture of acetic acid andwater with an acetic acid content of between 15% and 99% by weight,preferably between 15% and 80% by weight. More particularly, in the casewhere the diamine portion of the polyimide is 1,3-diaminobenzene, thepreferred water content in the coagulant is between 40% and 85% byweight when the solvent mixture contains N-methyl 2-pyrrolidone, between50% and 85% by weight when the solvent mixture containsN,N-dimethylacetamide, and between 55% and 85% by weight when thesolvent mixture contains gamma-butyrolactone. 1% to 5% by weight offormic acid and/or propanoic acid may optionally be added to thecoagulant. It may also contain small quantities of methanol and/orethanol, preferably less than a fifth of the weight of the water. It canbe filtered before use, for example, to about 0.2 micrometers.

Advantageously, the coagulated membrane is washed in a bath of water ora low alcohol such as methanol or ethanol before removal, if necessary,of the polyolefin protection. The residence time for the membrane in thewashing bath is preferably more than 10 seconds and can, for example, beas long as 5 minutes.

The membrane is dried in a current of air at a temperature of betweenroom temperature and about 50° C. The membrane is then heat treated,preferably in a low vacuum with increasing temperature, up to a value ofbetween 90° C. and 350° C., preferably between 110° C. and 200° C.

Finally, the membrane can be coated on its skin side with a thinsilicone elastomer layer. The membrane skin is brought into contact, fora period of several seconds to several minutes, with a commerciallyavailable formulation (such as Sylgard 184® from DOW CORNING) which isdiluted in a light saturated hydrocarbon such as pentane which isevaporated off at the end of the operation.

Asymmetric membranes of the invention are remarkable not only for theirhigh separation factors and selectivities in gases at room temperature,but also for their physical stability at higher temperatures which meansthat they are effective in the separation of hot gases.

In this respect, preferred membranes are those which are constituted bya polyimide which has been synthesized from 6F dianhydride and1,3-diaminobenzene. These membranes can have CO₂ /CH₄ theoreticalseparation factors of about 100 to 110 at room temperature and about 75to 85 at 55° C., and selectivities of about 80 to 90 at room temperatureand about 40 to 70 at 55° C. The CO₂ permeances of these preferredmembranes can be of the order of 2.10⁻¹⁰ to 4.5.10⁻¹⁰ Nm³.m⁻².s⁻¹.Pa⁻¹(ie., about 2.7.10⁻⁵ to 6.0.10⁻⁵ Ncm³.cm².s⁻¹.cmHg⁻¹) at roomtemperature and about 2.5.10⁻¹⁰ to 6.10⁻¹⁰ Nm³.m⁻².s⁻¹.Pa⁻¹ (ie., about3.3.10⁻⁵ to 8.0.10⁻⁵ Ncm³.cm².s⁻¹.cmHg⁻¹) at 55° C.

The following examples are given by way of illustration of the inventionand do not in any way limit the scope thereof. Gas permeancemeasurements in gas mixtures were carried out under the followingconditions. The skin side of the membrane was swept with a current ofpressurised gas mixture at a flow rate which was sufficient to be ableto assume that the composition of the mixture was negligibly affected bythe permeate stream. This flow rate, determined by experiment, was suchthat the partial flow rate of the gas which permeated most rapidly (CO₂,H₂ or O₂) was 50 times the partial flow rate of the same gas in thepermeate stream.

The permeate composition was determined by gas chromatography. Partialpressure differentials of each gas were used to calculate the permeancesof the gases in the mixture. The separation factors (theoretical) andselectivities (effective) were the ratios of the permeances of the twogases measured respectively in pure gas and in the gas mixture.

EXAMPLES 1-3

Examples 1 to 3 illustrate the production of flat 6F/mPDA polyimideasymmetric membranes under different solvent medium conditions forpolymer synthesis and for preparation of the coagulating solution. Thecomposition of the coagulant was different in each case.

In example 1, a fluorinated aromatic polyimide with a repeating unithaving formula III: ##STR3## was prepared using a chemical imidisationmethod. 177.70 g of 6F dianhydride (formula I) was gradually added to43.26 g of 1,3-diaminobenzene in 700 ml of N-methyl 2-pyrrolidone at 35°C. and under nitrogen, with stirring. Stirring was continued for 1 hourafter addition was complete. 700 g of a mixture of equal weights ofacetic anhydride and triethylamine was then introduced, with continuedstirring. This stirring was continued for 3 further hours. The polyimidewas precipitated out by pouring the solution into several times itsvolume of vigorously stirred ethanol. This was filtered, the precipitatewas washed and then vacuum dried for 12 hours at room temperature, then4 hours at 60° C. and finally for 2 hours at 150° C. Analysis of thesolid obtained indicated that it still contained 0.5% by weight ofN-methyl 2-pyrrolidone. The polyimide had an inherent viscosity of 0.80dl/g.

The polyimide was dissolved, under helium and with gentle stirring, in amixture of 1,4-dioxane and N-methyl 2-pyrrolidone which had beenpredried with 3A molecular sieve, to produce a solution with thefollowing composition by weight: 6F/mPDA polyimide: 22%; 1,4-dioxane:74%; N-methyl 2-pyrrolidone: 4% including the residual amount in thepolymer. The solution was filtered to 0.2 μm under pressurised heliumthen allowed to degas under helium at atmospheric pressure for 12 hoursin a vessel which was sufficiently wide for the liquid to be less than 5cm deep.

A film of the solution with a thickness of about 150 μm was deposited,using a blade applicator, on a commercially available nonwoven polyesterfabric band with a thickness of 100 μm and width of 1 m advancing at arate of 3 m per minute. The film was exposed for 6 seconds to atangential 300 1 per minute stream of dry nitrogen circulating in theopposite direction to that of the film, between the film and a wallplaced 1 cm therefrom. The wall was flanged at its edges to prevent thenitrogen stream from reaching the face of the film against the nonwovenfabric.

The film was then immersed for 15 seconds in a coagulant bath consistingof a mixture of 52% by weight acetic acid in water, filtered to 0.2 μm.The mixture was continuously changed at a rate of 5 1 per minute. Thefilm, which had become an asymmetric membrane, was washed for 20 secondsin a counter-current of ethanol hydrated to about 10% by weight ofwater, changed at a rate of 5 1 per minute. The extrusion, coagulationand washing operations were carried out at about 20° C. The membranethen passed through a drying tunnel using dry air at 40° C. before beingrolled onto a metal bobbin. The bobbin was placed in a vacuum oven wherethe membrane was exposed to a temperature of 150° C. for 12 hours.Finally, the membrane skin was treated conventionally by coating it witha silicone elastomer layer. The membrane was unrolled horizontally fromthe bobbin with the skin side downwards. The skin was sprayed with a 7%by weight solution in pentane of Sylgard 184® from DOW CORNING inpentane. A 3 centimetre margin at the edges was coated with a Teflon®mask. This margin was used to seal the membrane during the manufactureof conventional spiral. type modules. The membrane was dried at 40° C.in a slow stream of nitrogen before being rolled up again onto a furtherbobbin.

The permeances, separation factors and selectivities of the membranewere evaluated using measurements carried out in pure carbon dioxide(CO₂), pure hydrogen (H₂), pure methane (CH₄), a CO₂ --CH₄ mixturesaturated with water vapour, a H₂ --CH₄ mixture and finally a O₂ --N₂mixture. Measurements on the mixture of CO₂ and CH₄ were carried outunder two different pressure and temperature conditions, combiningrelatively high temperatures and high partial pressures of CO₂.

The results are shown in Table 1 below.

Example 2 differs from Example 1 in using N,N-dimethylacetamide as thesolvent both for the synthesis of the polyimide and for the preparationof the coagulating solution, which also modified the composition of thecoagulant. The inherent viscosity of the polyimide was 0.75 dl/g. Thepolymer obtained had a residual solvent content of 0.7% by weight. Thecomposition by weight of the coagulating solution was as follows:6F/mPDA polyimide: 22%; 1,4-dioxane: 74%; N,N-dimethylacetamide: 4%. Thecomposition by weight of the coagulant was 55% acetic acid and 45%water.

Permeances, separation factors and selectivities of the membrane towardsthe gases are given in Table 2 below.

Example 3 differs from Example 1 only in that gamma-butyrolactone wasused to prepare the polyimide solution for coagulation. The synthesissolvent and the coagulant were unchanged. The coagulating solvent, whichcontained a little of the synthesizing solvent carried by the polymer,had the following composition by weight: 6F/mPDA polyimide: 22%;1,4-dioxane: 74%; gamma-butyrolactone: 3.9%; N-methyl 2-pyrrolidone:0.1%.

Membrane permeances and selectivities in the gases are given in Table 3below.

EXAMPLES 4 AND 5

These examples illustrate the production of flat asymmetric membranesfrom 6F/mPDA polyimides which have been monomethylated on the aromaticring of the diamine. These polyimides had far higher permeabilities thanthat of 6F/mPDA polyimide but, on the other hand, had lowerselectivities.

Example 4 concerns an asymmetric membrane constituted by a polyimideobtained by polycondensation of 6F dianhydride and 2,6-diaminotoluene or2,6-toluenediamine, abbreviated to 2,6-TDA, with a repeating unit havingformula IV: ##STR4##

Synthesis of this polyimide was as described for Example 1 except that48.87 g of 2,6-diaminotoluene was reacted with 177.70 g of 6Fdianhydride. The polyimide had an inherent viscosity of 0.83 dl/g. Themembrane was formed as in Example 1 except that the coagulant wascomposed of 55% by weight of acetic acid and 45% by weight of water. Gaspermeances and selectivities of the membrane are shown in Table 4 below.

Example 5 concerns a flat asymmetric membrane constituted by a polyimideobtained by polycondensation of 6F dianhydride and 2,4-diaminotoluene or2,4-toluenediamine, abbreviated to 2,4-TDA, with a repeating unit havingformula V: ##STR5##

Synthesis of this polyimide was as described for Example 1 except that54.48 g of 2,4-diaminotoluene was reacted with 177.70 g of 6Fdianhydride.

The polyimide had an inherent viscosity of 0.91 dl/g. The membrane wasformed as in Example 1 except that the coagulant was composed of 57% byweight of acetic acid and 43% by weight of water. Gas permeances andselectivities of the membrane are shown in Table 5 below.

COMPARATIVE EXAMPLES 1-22

These examples illustrate the mediocre gas separation capability, due tolack of selectivity and/or permeability, of flat asymmetric membranesproduced by methods which only differed from the method of the invention(Example 1) in the nature of the polyimide solvent and, whereappropriate, that of the coagulant. The results are shown in Table 6below.

COMPARATIVE EXAMPLES 23-26

These examples illustrate the effect on gas separation capability offlat asymmetric membranes produced by methods which only differed fromthe method of the invention (Example 1) in the chemical nature of thecoagulant. The results, shown in Table 7 below, show very largereductions in permeance.

COMPARATIVE EXAMPLES 27-30

These examples illustrate the effect on gas separation capability offlat asymmetric membranes produced by methods which only differed fromthe method of the invention (Example 1) in the proportions of componentsin the coagulant. The results are shown in Table 8 below. They indicatea rapid reduction in selectivity when the acetic acid content of theaqueous acetic acid solution goes above the maximum value defined in themethod of the invention. The results also show a rapid reduction inpermeance when the acetic acid content of the aqueous acetic acidsolution drops below the minimum value defined in the method of theinvention.

COMPARATIVE EXAMPLES 31 AND 32

These examples illustrate the effect on gas separation capability offlat asymmetric membranes produced by methods which only differed fromthe method of the invention (Example 1) in the proportions of thecomponents of the polyimide solvent. The results are shown in Table 9below. They show a rapid reduction in selectivity when the 1,4-dioxanecontent of the solvent drops below the minimum value defined in themethod of the invention.

EXAMPLES 6 AND 7

Examples 6 and 7 illustrate production of fluorinated aromatic polyimideasymmetric membranes in the from of hollow fibres.

Example 6 concerns a hollow asymmetric 6F/mPDA polyimide fibre.

The polyimide was synthesized and the coagulating solution was preparedas described for Example 1, including the quantities of polymer andsolution. This had a composition by weight of 22% of polyimide, 74% of1,4-dioxane and 4% of N-methyl 2-pyrrolidone. In addition, a stock ofcentral liquid was made by mixing 50 g of acetic acid, 50 g of water and25 g of 1,4-dioxane and filtering the mixture to 0.2 μm. A die with thefollowing characteristics was used: external diameter of annularorifice: 0.6 mm; internal diameter of annular orifice: 0.2 mm; centralorifice diameter: 0.19 mm. The extremity of the separation tube for thetwo orifices was located 1 mm forward of the extremity of the externalwall of the annular orifice.

The coagulation bath was filled with 20 1 of coagulant prepared as inExample 1 and constituted by an aqueous solution of 52% by weight aceticacid. The height of the liquid in the bath was 1.2 m. The coagulant wascontinuously changed at a rate of 0.5 l per minute. The extremity of thedie was positioned 5 cm above the coagulant level. It was positioned ina vertical glass tube of 2 cm internal diameter which opened 5 mm abovethe coagulant surface. The hollow fibre was formed under the followingconditions: the polyimide solution was extruded at a rate of 200 ml perhour while the central liquid was emitted at a rate of 30 ml per hourand dry nitrogen circulated around the extrudate in a descending streamat a rate of 92 l per hour. The extrudate stayed in the coagulant for 15seconds before emerging vertically in the form of a massive flexiblehollow fibre pulled at a rate which was sufficient to avoid itsaccumulation in the bath. The hollow fibre was then washed for 20seconds in a counter-current of ethanol containing about 10% by weightof water, changed at a rate of 0.5 l per minute. The extrusion,coagulation and washing operations were carried out at about 20° C. Thewashed fibre passed through a drying tunnel of dry air at 40° C. beforebeing deposited in a cylindrical metal receptacle. The receptacle wasplaced in a vacuum oven where the membrane was exposed to a temperatureof 150° C. for 12 hours.

Finally, the membrane was conventionally coated with a layer of siliconeelastomer. This was carried out by passing the hollow fibre, taken up ona winding means, through a bath of a solution in pentane of 7% by weightof Sylgard 184® from DOW CORNING and drying in a slow stream of nitrogenat 40° C.

Permeances and selectivities of the membrane, measured in mixed gases,are shown in Table 10 below.

Example 7 concerns an asymmetric hollow 6F/2,6-TDA polyimide fibre.

Synthesis of the polyimide was carried out as described in Example 4 andthe membrane was manufactured as described in Example 5.

Permeances and selectivities for the membrane, measured for gasmixtures, are shown in Table 11 below.

                                      TABLE 1                                     __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   58     56     75                                                      Permeance* CH.sub.4                                                                   0.53                                                                  Theoretical                                                                           109                                                                   Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                     90     60                                                      Permeance* H.sub.2            210                                             Sel'y H.sub.2 /CH.sub.4       370                                             Permeance* O.sub.2                    18                                      Sel'y O.sub.2 /H.sub.2                7.1                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 2                                     __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   61     59     77                                                      Permeance* CH.sub.4                                                                   0.57                                                                  Theoretical                                                                           107                                                                   Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                     88     57                                                      Permeance* H.sub.2            220                                             Sel'y H.sub.2 /CH.sub.4       355                                             Permeance* O.sub.2                    20                                      Sel'y O.sub.2 /H.sub.2                6.7                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 3                                     __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   59     52     72                                                      Permeance* CH.sub.4                                                                   0.55                                                                  Theoretical                                                                           108                                                                   Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                     89     59                                                      Permeance* H.sub.2            205                                             Sel'y H.sub.2 /CH.sub.4       360                                             Permeance* O.sub.2                    16                                      Sel'y O.sub.2 /H.sub.2                7.0                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 4                                     __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   177    170    240                                                     Permeance* CH.sub.4                                                                   2.3                                                                   Theoretical                                                                           77                                                                    Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                      60     42                                                     Permeance* H.sub.2            420                                             Sel'y H.sub.2 /CH.sub.4       140                                             Permeance* O.sub.2                    77                                      Sel'y O.sub.2 /H.sub.2                6.6                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 5                                     __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   197    190    270                                                     Permeance* CH.sub.4                                                                   2.7                                                                   Theoretical                                                                           73                                                                    Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                      57     40                                                     Permeance* H.sub.2            560                                             Sel'y H.sub.2 /CH.sub.4       135                                             Permeance* O.sub.2                    90                                      Sel'y O.sub.2 /H.sub.2                6.5                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 6                                     __________________________________________________________________________    (comparative examples not in accordance with the invention)                                                             Permeametry at 20° C. in                                               gas mixtures                                                                  Upstream pressure: 7 MPa                                       Permeametry at 20° C.                                                                 Downstream pressure: 0.11 MPa                                  with pure gases                                                                              CO.sub.2 + CH.sub.4                                       Phys'l                                                                             Upstream pressure: 0.8 MPa                                                                   mixture    H.sub.2 + CH.sub.4                        Filtrat'n                                                                          char's                                                                             Downstream pressure .sup.˜  0.05                                                       30-70% mol mixture                                   quality                                                                            of   Per- Per- CO.sub.2 /CH.sub.4                                                                 (water saturated)                                                                        20-80% mol               Ex                                                                              Poly-          of   filtered                                                                           meance*                                                                            meance*                                                                            Sep'n                                                                              Permeance*                                                                          CO.sub.2 /CH.sub.4                                                                 Permeance*                                                                          H.sub.2                                                                       /CH.sub.4          No                                                                              imide                                                                             Solvent                                                                             Coagulant                                                                          solution                                                                           solution                                                                           to CO.sub.2                                                                        to CH.sub.4                                                                        Factor                                                                             to CO.sub.2                                                                         sel'y                                                                              to                                                                                  sel'y.2            __________________________________________________________________________    1 6F/ N-methyl                                                                            water                                                                              0.5 μm                                                                          clear                                                                              12   0.26 47   12    38   47    136                  mPDA                                                                              2-pyrrol-       no gel                                                        idone                                                                   2 6F/ N-methyl                                                                            methanol                                                                           0.5 μm                                                                          clear                                                                              14   0.31 45   14    37   55    125                  mPDA                                                                              2-pyrrol-       no gel                                                        idone                                                                   3 6F/ N-methyl                                                                            ethanol                                                                            0.5 μm                                                                          clear                                                                              13   0.28 46   13    37   49    130                  mPDA                                                                              2-pyrrol-       no gel                                                        idone                                                                   4 6F/ N-methyl                                                                            50/50                                                                              0.5 μm                                                                          clear                                                                              17   0.40 42   16    35   69    108                  mPDA                                                                              2-pyrrol-                                                                           wt %      no gel                                                        idone acetic                                                                        acid +                                                                        water                                                             5 6F/ NNdimethyl                                                                          water                                                                              0.5 μm                                                                          clear                                                                              14   0.33 43   14    36   56    118                  mPDA                                                                              acetamide       no gel                                                  6 6F/ NNdimethyl                                                                          methanol                                                                           0.05 μm                                                                         clear                                                                              18   0.44 41   17    35   68    112                  mPDA                                                                              acetamide       no gel                                                  7 6F/ NNdimethyl                                                                          50/50                                                                              0.5 μm                                                                          clear                                                                              20   0.50 40   19    34   71    103                  mPDA                                                                              acetamide                                                                           wt %      no gel                                                              acetic                                                                        acid +                                                                        water                                                             8 6F/ γ-butyro-                                                                     water                                                                              0.5 μm                                                                          clear                                                                              10   0.21 48   10    39   41    142                  mPDA                                                                              lactone         no gel                                                  9 6F/ γ-butyro-                                                                     methanol                                                                           0.5 μm                                                                          clear                                                                              10   0.22 46   10    38   43    140                  mPDA                                                                              lactone         no gel                                                  10                                                                              6F/ γ-butyro-                                                                     50/50                                                                              0.5 μm                                                                          clear                                                                              12   0.29 42   12    35   47    133                  mPDA                                                                              lactone                                                                             wt %      no gel                                                              acetic                                                                        acid +                                                                        water                                                             11                                                                              6F/ tetra-                                                                              water                                                                              0.5 μm                                                                          clear                                                                              7    0.16 45   7     37   27    120                  mPDA                                                                              hydrofuran      beginning                                                                     to gel                                                  12                                                                              6F/ tetra-                                                                              methanol                                                                           0.5 μm                                                                          clear                                                                              79   1.8  44   77    36   29    115                  mPDA                                                                              hydrofuran      beginning                                                                     to gel                                                  13                                                                              6F/ tetra-                                                                              50/50                                                                              0.5 μm                                                                          clear                                                                              80   2.1  39   79    33   34    101                  mPDA                                                                              hydrofuran                                                                          wt %      beginning                                                           acetic    to gel                                                              acid +                                                                        water                                                             14                                                                              6F/ acetone                                                                             water                                                                                1 μm                                                                          clear                                                                              77   1.7  45   75    36   20    125                  mPDA                beginning                                                                     to gel                                                  15                                                                              6F/ acetone                                                                             methanol                                                                             1 μm                                                                          clear                                                                              6    0.14 44   6     36   22    120                  mPDA                beginning                                                                     to gel                                                  16                                                                              6F/ acetone                                                                             50/50                                                                                1 μm                                                                          clear                                                                              8    0.20 40   8     34   28    105                  mPDA      wt %      beginning                                                           acetic    to gel                                                              acid +                                                                        water                                                             17                                                                              6F/ N-methyl                                                                            water                                                                              0.5 μm                                                                          clear                                                                              28   0.70 40   27    34   85    54                   26TDA                                                                             2-pyrrol-       no gel                                                        idone                                                                   18                                                                              6F/ N-methyl                                                                            methanol                                                                           0.5 μm                                                                          clear                                                                              32   0.84 38   31    33   96    50                   26TDA                                                                             2-pyrrol-       no gel                                                        idone                                                                   19                                                                              6F/ N-methyl                                                                            50/50                                                                              0.5 μm                                                                          clear                                                                              38   1.2  32   36    30   121   44                   26TDA                                                                             2-pyrrol-                                                                           wt %      no gel                                                        idone acetic                                                                        acid +                                                                        water                                                             20                                                                              6F/ N-methyl                                                                            water                                                                              0.5 μm                                                                          clear                                                                              42   1.1  39   40    33   97    52                   24TDA                                                                             2-pyrrol-       no gel                                                        idone                                                                   21                                                                              6F/ N-methyl                                                                            methanol                                                                           0.5 μm                                                                          clear                                                                              46   1.2  38   45    31   115   47                   24TDA                                                                             2-pyrrol-       no gel                                                        idone                                                                   22                                                                              6F/ N-methyl                                                                            50/50                                                                              0.5 μm                                                                          clear                                                                              50   1.4  36   48    30   137   42                   24TDA                                                                             2-pyrrol-                                                                           wt %      no gel                                                        idone acetic                                                                        acid +                                                                        water                                                             __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 · 10.sup.-12 Nm.sup.3 ·       m.sup.-2 · s.sup.-1 · Pa.sup.-1                        

                                      TABLE 7                                     __________________________________________________________________________    (comparative examples not in accordance with the invention)                                                             Permeametry at 20° C. in                                               gas mixtures                                                                  Upstream pressure: 7 MPa                                       Permeametry at 20° C.                                                                 Downstream pressure: 0.11 MPa                                  with pure gases                                                                              CO.sub.2 + CH.sub.4                                       Phys'l                                                                             Upstream pressure: 0.8 MPa                                                                   mixture    H.sub.2 + CH.sub.4                        Filtrat'n                                                                          char's                                                                             Downstream pressure .sup.˜  0.05                                                       30-70% mol mixture                                   quality                                                                            of   Per- Per- CO.sub.2 /CH.sub.4                                                                 (water saturated)                                                                        20-80% mol               Ex                                                                              Poly-          of   filtered                                                                           meance*                                                                            meance*                                                                            Sep'n                                                                              Permeance*                                                                          CO.sub.2 /CH.sub.4                                                                 Permeance*                                                                          H.sub.2                                                                       /CH.sub.4          No                                                                              imide                                                                             Solvent                                                                             Coagulant                                                                          solution                                                                           solution                                                                           to CO.sub.2                                                                        to CH.sub.4                                                                        Factor                                                                             to CO.sub.2                                                                         sel'y                                                                              to                                                                                  sel'y.2            __________________________________________________________________________    23                                                                              6F/ 1,4-  water                                                                              0.5 μm                                                                          clear                                                                              12   0.31 39   12    33   45    110                  mPDA                                                                              dioxane +       no gel                                                        N-methyl                                                                      2-pyrrol-                                                                     idone                                                                         95/5 wt %                                                               24                                                                              6F/ 1,4-  methanol                                                                           0.5 μm                                                                          clear                                                                              15   0.37 40   14    35   51    125                  mPDA                                                                              dioxane +       no gel                                                        N-methyl                                                                      2-pyrrol-                                                                     idone                                                                         95/5 wt %                                                               25                                                                              6F/ 1,4-  ethanol                                                                            0.5 μm                                                                          clear                                                                              20   0.49 41   19    35   72    130                  mPDA                                                                              dioxane +       no gel                                                        N-methyl                                                                      2-pyrrol-                                                                     idone                                                                         95/5 wt %                                                               26                                                                              6F/ 1,4-  n-heptane                                                                          0.5 μm                                                                          clear                                                                              10   0.20 51   10    42   38    155                  mPDA                                                                              dioxane +       no gel                                                        N-methyl                                                                      2-pyrrol-                                                                     idone                                                                         95/5 wt %                                                               __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 · 10.sup.-12 Nm.sup.3 ·       m.sup.-2 · s.sup.-1 · Pa.sup.-1                        

                                      TABLE 8                                     __________________________________________________________________________    (comparative examples not in accordance with the invention)                                                             Permeametry at 20° C. in                                               gas mixtures                                                                  Upstream pressure: 7 MPa                                       Permeametry at 20° C.                                                                 Downstream pressure: 0.11 MPa                                  with pure gases                                                                              CO.sub.2 + CH.sub.4                                       Phys'l                                                                             Upstream pressure: 0.8 MPa                                                                   mixture    H.sub.2 + CH.sub.4                        Filtrat'n                                                                          char's                                                                             Downstream pressure .sup.˜  0.05                                                       30-70% mol mixture                                   quality                                                                            of   Per- Per- CO.sub.2 /CH.sub.4                                                                 (water saturated)                                                                        20-80% mol               Ex                                                                              Poly-          of   filtered                                                                           meance*                                                                            meance*                                                                            Sep'n                                                                              Permeance*                                                                          CO.sub.2 /CH.sub.4                                                                 Permeance*                                                                          H.sub.2                                                                       /CH.sub.4          No                                                                              imide                                                                             Solvent                                                                             Coagulant                                                                          solution                                                                           solution                                                                           to CO.sub.2                                                                        to CH.sub.4                                                                        Factor                                                                             to CO.sub.2                                                                         sel'y                                                                              to                                                                                  sel'y.2            __________________________________________________________________________    27                                                                              6F/ 1,4-  acetic                                                                             0.5 μm                                                                          clear                                                                              54   1.5  36   52    30   130   12                   mPDA                                                                              dioxane +                                                                           acid +    no gel                                                        N-methyl                                                                            water                                                                   2-pyrrol-                                                                           98/2 wt %                                                               idone                                                                         95/5 wt %                                                               28                                                                              6F/ 1,4-  acetic                                                                             0.5 μm                                                                          clear                                                                              46   0.78 59   43    49   163   128                  mPDA                                                                              dioxane +                                                                           acid +    no gel                                                        N-methyl                                                                            water                                                                   2-pyrrol-                                                                           90/10                                                                   idone wt %                                                                    95/5 wt %                                                               29                                                                              6F/ 1,4-  acetic                                                                             0.5 μm                                                                          clear                                                                              30   0.54 56   29    47   109   163                  mPDA                                                                              dioxane +                                                                           acid +    no gel                                                        N-methyl                                                                            water                                                                   2-pyrrol-                                                                           15/85                                                                   idone wt %                                                                    95/5 wt %                                                               30                                                                              6F/ 1,4-  acetic                                                                             0.5 μm                                                                          clear                                                                              24   0.48 50   23    41   84    149                  mPDA                                                                              dioxane +                                                                           acid +    no gel                                                        N-methyl                                                                            water                                                                   2-pyrrol-                                                                           5/95 wt %                                                               idone                                                                         95/5 wt %                                                               __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 · 10.sup.-12 Nm.sup.3 ·       m.sup.-2 · s.sup.-1 · Pa.sup.-1                        

                                      TABLE 9                                     __________________________________________________________________________    (comparative examples not in accordance with the invention)                                                             Permeametry at 20° C. in                                               gas mixtures                                                                  Upstream pressure: 7 MPa                                       Permeametry at Downstream pressure: 0.11 MPa                                  20° C. with pure gases                                                                CO.sub.2 + CH.sub.4                                        Phys'l                                                                            Upstream pressure: 0.8 MPa                                                                   mixture    H.sub.2 + CH.sub.4                          Filtrat'n                                                                         char's                                                                            Downstream pressure .sup.˜ 0.05                                                        30-70% mol mixture                                     quality                                                                           of  Perm-                                                                             Perm-      (water saturated)                                                                        20-80% mol               Ex                                                                              Poly-            of  filtered                                                                          eance*                                                                            eance*                                                                             CO.sub.2 /CH.sub.4                                                                  Permeance*                                                                          CO.sub.2 /CH.sub.4                                                                 Permeance*                                                                          H.sub.2                                                                       /CH.sub.4          No                                                                              imide                                                                             Solvent                                                                              Coagulant                                                                           solution                                                                          solution                                                                          to CO.sub.2                                                                       to CH.sub.4                                                                        Sep'n Factor                                                                        to CO.sub.2                                                                         sel'y                                                                              to                                                                                  sel'y.2            __________________________________________________________________________    31                                                                              6F/ 1,4-dioxane +                                                                        acetic                                                                              0.5 μm                                                                         clear                                                                             15  0.33 45    15    38   56    137                  mPDA                                                                              N-methyl                                                                             acid +    no gel                                                       2-pyrrol-                                                                            water                                                                  idone  50/50 wt %                                                             70/30 wt %                                                              32                                                                              6F/ 1,4-dioxane +                                                                        acetic                                                                              0.5 μm                                                                         clear                                                                             23  0.45 51    22    43   79    168                  mPDA                                                                              N-methyl                                                                             acid +    no gel                                                       2-pyrrol-                                                                            water                                                                  idone  50/50 wt %                                                             80/20 wt %                                                              __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                 

                                      TABLE 10                                    __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   57     51     68                                                      Permeance* CH.sub.4                                                                   0.52                                                                  Theoretical                                                                           110                                                                   Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                     96     59                                                      Permeance* H.sub.2            200                                             Sel'y H.sub.2 /CH.sub.4       360                                             Permeance* O.sub.2                    17                                      Sel'y O.sub.2 /H.sub.2                7.0                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

                                      TABLE 11                                    __________________________________________________________________________                                  Mixture Mixture                                         Pure Gases                                                                           Mixture (water saturated)                                                                    H.sub.2 (20% mol) +                                                                   O.sub.2 (20% mol) +                             (CO.sub.2 and CH.sub.4)                                                              CO.sub.2 (30% mol) + CH.sub.4 (70% mol)                                                      CH.sub.4 (80% mol)                                                                    N.sub.2 (80% mol)                               Temp: 20° C.                                                                  Temp: 20° C.                                                                  Temp: 55° C.                                                                   Temp: 20° C.                                                                   Temp: 20° C.                             Pr***: 0.8 MPa                                                                       Pr**: 2.5 MPa                                                                        Pr**: 7 MPa                                                                           Pr**: 4 MPa                                                                           Pr**: 3 MPa                             __________________________________________________________________________    Permeance* CO.sub.2                                                                   162    155    220                                                     Permeance* CH.sub.4                                                                   2.2                                                                   Theoretical                                                                           74                                                                    Sep'n Factor                                                                  CO.sub.2 /CH.sub.4                                                            Sel'y CO.sub.2 /CH.sub.4                                                                      59     41                                                     Permeance* H.sub.2            380                                             Sel'y H.sub.2 /CH.sub.4       135                                             Permeance* O.sub.2                    70                                      Sel'y O.sub.2 /H.sub.2                6.5                                     __________________________________________________________________________     *in 10.sup.-6 Ncm.sup.3 · cm.sup.-2 · s.sup.-1              · cmHg.sup.-1 or 7.5 10.sup.-12 Nm.sup.3 · m.sup.-2         · s.sup.-1 · Pa.sup.-1                                      **upstream pressure (downstream pressure 0.11 MPa)                            ***upstream pressure (downstream pressure .sup.˜ 0.05 MPa)         

I claim:
 1. An asymmetric, aromatic polyimide or copolyimide membranefor gas separation, containing, as monomers,diphenyldi(trifluoromethyl)methane 3,4,3',4'4'-tetracarboxylic aciddianhydride and a monoaromatic metadiamine or a mixture of monoaromaticmetadiamines, said membrane comprising a dense skin and a poroussubstructure and having a theoretical separation factor of about 70 to110 and an effective selectivity of about 50 to 90 at room temperaturein a CO₂ /CH₄ couple.
 2. Membrane according to claim 1 wherein themonoaromatic metadiamine is 1,3-diaminobenzene.
 3. Membrane according toclaim 2 having a theoretical separation factor of about 100 to 110 and aselectivity of about 80 to 90 at room temperature for the CO₂ /CH₄couple.
 4. Membrane according to claim 3 having a theoretical separationfactor of about 75 to 85 and a selectivity of about 40 to 70 at 55° C.for the CO₂ /CH₄ couple.
 5. Membrane according to claim 1 coated on theskin side with a silicone elastomer layer.
 6. An asymmetric, rigidaromatic polyimide or copolyimide membrane for gas separation,containing, as monomers,diphenyldi(trifluoromethyl)methane3,4,3',4'-tetracarboxylicaciddianhydrideand a monoaromatic metadiamine, said membrane comprising a dense skinand a porous substructure and having a theoretical separation factor ofabout 70 to 110 and an effective selectivity of about 50 to 90 at roomtemperature in a CO₂ /CH₄ couple, wherein the monoaromatic metadiamineis a monomethylated derivative of 1,3-diaminobenzene.
 7. An asymmetric,rigid aromatic polyimide or copolyimide membrane for gas separation,containing, as monomers,diphenyldi(trifluoromethyl)methane3,4,3',4'-tetracarboxylicaciddianhydrideand a monoaromatic metadiamine, said membrane comprising a dense skinand a porous substructure and having a theoretical separation factor ofabout 70 to 110 and an effective selectivity of about 50 to 90 at roomtemperature in a CO₂ /CH₄ couple, wherein the monoaromatic metadiamineis a dimethylated derivative of 1,3-diaminobenzene.